For high resolution mass spectrometers, it is desirable that ions for analysis have a low emittance, since this improves resolving power and sensitivity. The ion beam emittance is in part determined by its temperature. Traditionally, ions are cooled by collisions with a gas. However, since the mass analyser operates at a high vacuum, it is desirable to cool the ions outside of the mass analyser.
Existing solutions have followed two divergent strategies. A first strategy is to cool the ions in a trap, external to the mass analyser, which contains a gas suitable for cooling the ions, for example as shown in U.S. Pat. No. 6,674,071. However, in such devices, the ions can only be cooled to the temperature of the gas, which is limited to that of the ion optics of the trap, since the gas is effectively held within the trap, in thermal equilibrium with the material with which it is trapped. This places a limit on the minimum achievable emittance of the cooled ion beam.
Moreover, when the trap is used for injecting the cooled ions into the mass spectrometer, further problems arise. Accelerating the ions through the gas can cause fragmentation of the ions and removing the gas before ejecting the ions limits the speed of operation significantly. If the ions are accelerated using electric fields, as is typically done, although all ions are accelerated to a constant energy, their velocity will depend on their mass. Hence, the path length from trap to analyser must be minimised in such cases, to mitigate time-of-flight mass separation outside of the analyser. This turns prevention of gas carry-over into the analyser into a serious problem. This problem is typically alleviated by avoiding line-of-sight between the trap and the analyser and by using small apertures, at the expense of increased cost and complexity and reduced performance. Also, this further increases path length, thereby worsening any external mass separation problems.
In an implementation of this strategy, it is known to introduce the gas as pulses. This assists in reducing gas load during analysis after ion cooling. Older examples of this include: J. Carlin and B. S. Freiser, Anal. Chem. 55 (1983), 571; B. Emary, R. E. Kaiser, H. I. Kenttamaa and R. G. Cooks, J. Am. Soc. Mass Spectrom. 1 (1990) 308; and R. C. Beavis and B. T. Chait, Chem. Phys. Lett. 181 (1991) 479.
Other, more recent examples of this technique include: GB-2439107; and D. Papanastasiou, O. Belgacem, M. Sudakov and E. Raptakis, Rev. Sci. Instrum. 79 (2008) 055103. However, there is no fundamental difference between this technique and the static operation. In both techniques, gas enters the ion trap in a highly diffused manner.
The second strategy has been to direct the ion beam through a gas jet. For example, U.S. Pat. No. 5,373,156 describes a method for cooling very heavy ions (300,000 to 2 million Da) using a light gas, such as hydrogen or helium, which is adiabatically cooled during formation and directed as a jet. The gas jet also causes the ions to decelerate immediately upstream of the mass analyser, thereby avoiding potential fragmentation or mass separation problems. However, such devices are not compatible with trapping, since they are designed to allow the cooled ions to be injected into the mass analyser immediately. Furthermore, gas carry-over remains a problem with such devices.
Gas jets have been used for collision induced dissociation (CID) of ions. In U.S. Pat. No. 4,328,420, a mass spectrometer is disclosed having three quadrupole sections with the middle section acting as a collision cell with a gas jet intercepting the ion beam in an orthogonal manner. Similar arrangements for CID with a gas jet entering a collision cell and intercepting an ion path in an orthogonal manner are disclosed in US 2004/0119015 and US 2007/0085000, including arrangements with an ion trap. Such ion traps additionally employ a significant pressure of bath gas in the trap for storage of the ions thus significantly adding to the gas load of the system.